The present invention relates to methods, compositions, and apparatuses for the detection and prevention of chatter in doctor blades on a Yankee dryer. As described at least in U.S. Pat. Nos. 7,691,236, 7,850,823, 5,571,382, 5,187,219, 5,179,150, 5,123,152, 4,320,582, and 3,061,944, in the tissue manufacturing process, a paper sheet is dried on a heated drying cylinder, termed a Yankee or Yankee dryer. Often adhesive materials are used to coat the Yankee surface in order to help the wet sheet adhere to the dryer. This improves heat transfer, allowing more efficient drying of the sheet. Most importantly, these adhesives provide the required adhesion to give good creping of the dry sheet. Creping is the process of impacting the sheet into a hard blade (called a doctor blade), thus compressing the sheet in the machine direction, creating a folded sheet structure. Creping breaks a large number of fiber-to-fiber bonds in the sheet, imparting the qualities of bulk, stretch, absorbency, and softness which are characteristic of tissue. The amount of adhesion provided by the coating adhesive plays a significant role in the development of these tissue properties.
In addition, the present invention covers detection and prevention of chatter in doctor blades used for cleaning residual coating from the Yankee surface as well as the cut-off doctor blade used during maintenance operations on the creping doctor blade. The present invention focuses on the creping operation, but extension of methodology to the cleaning and cut-off blade apply equally as well.
The Yankee coating also serves the purpose of protecting the Yankee and creping blade surfaces from excessive wear. In this role, the coating agents provide improved runabitity of the tissue machine. As creping doctor blades wear, they must be replaced with new ones. The process of changing blades represents a significant source of tissue machine downtime, or lost production, as creped product cannot be produced when the blade is being changed. Release agents, typically hydrocarbon oils, are used in association with the coating polymers. These agents aid in the uniform release of the tissue web at the creping blades, and also lubricate and protect the blade from excessive wear.
Proper and sustained interaction between the Yankee coating and the creping doctor blade is critical for both sheet property development and machine runnability. In normal operations, the creping doctor blade tip rides in the coating on the dryer surface and experiences minimal out of plane movement. However, if the amplitude of the out of plane movement becomes high enough the creping doctor blade wilt oscillate above and below the sheet leading to the development of chatter that appears as cross directional (CD) defects. Sheet defects from chatter will appear as multiple holes in the CD or develop a lace appearance. Coating defects can exhibit long CD marks that are visible when viewed with a strobe light. Under severe chatter conditions, the doctor blade will penetrate through the Yankee coating making direct contact with the dryer surface. If this occurs, potential damage to the dryer surface with the appearance of horizontal grooves on the metal surface can result. Once the dryer surface becomes damaged, it can only be repaired by taking the machine out of production and regrinding the dryer surface. Regrinding is a costly operation, because of production losses and cost of the procedure as well as degrading the dryer service lifetime due to reduction in wall thickness that negatively affects the vessel pressure rating. Therefore, it is imperative for manufacturers to closely monitor the process and identify when chatter is present.
Excessive vibration on the creping doctor blade, leading to chatter conditions, can originate through mechanical and operational or process conditions. Examples of mechanical vibration sources include press rolls, pumps, felts, Yankee cylinder bearings, etc., as well as dryer roundness deformation caused by thermal non-uniformities. Once a mechanical vibration source is identified, maintenance intervention to correct the problem often requires shutting down the equipment resulting in production loss. Conversely, operational practices or process conditions inducing excess vibration may include sheet moisture levels, coating chemistry, machine speed, basis weight, furnish, blade stick out and loading pressure, etc. can be attended to without interrupting production.
Regardless of the source, excess vibration experienced by the doctor blade can lead to chatter conditions affecting product quality, machine runnability, and asset value. Operators will often rely on audible sound changes or visual inspection (sheet quality or Yankee dryer surface) as the first indication that chatter is present. However, this approach is subjective and not reliable often resulting in detecting chatter after the condition has become severe, thus making corrective action steps more difficult. To improve the reliability and detection sensitivity for chatter detection, condition monitoring (CM) technology using piezoelectric and/or microphone sensor(s) can be used. CM has a long history in the paper industry, but mainly for use in bearing monitoring on rotating components. Examples of using CM on the creping doctor blade is limited and in these cases measurement analysis is made following traditional CM methods based on sensor signal level exceeding an alarm limit. In this approach, the system state is assessed from the sensor signal trend. A flat trend is considered a normal condition whereas an upward sloping trend indicates a wear condition, and a step change is considered a component failure. The dynamics of the Yankee dryer operation can produce large variations in the sensor signal, without reaching a chatter condition. As a result, data analysis becomes more complex compared to conventional CM based on wear and failure detection levels.
Previous attempts to address this problem include: Aurelio Alessadrini and Piero Pagani, Chatter Marks: Origin, Evolution and Influence of the Creeping Doctors, Ind. Carta vol. 41, no. 4, June 2003, pp 120-129, S. Archer, V. Grigoriev, G. Furman, L. Bonday, and W. Su, Chatter and Soft Tissue Production: Process Driven Mechanisms, Tissue World Americas, February-March 2009, pp 33-35, S. Zhang, J. Mathew, L. Ma, Y Sun, and A. Mathew, Statistical condition monitoring based on vibration signals, Proceedings VETOMAC-3 & ACISM-2004, pp. 1238-1243, New Delhi, India, M Fugate, H Sohn, and C. Farrar, Vibration-based damage detection using statistical process control, Mechanical Systems and Signal Processing, Vol. 15, Issue 4, July 2001, pp 707-721, H Sohn, C. Farrar, Damage diagnosis using time series analysis of vibration signals, Smart Materials and Structures, Vol 10, 2001, pp. 446-451, A. Heng, S. Zhang, A. Tan, and J Mathew, Rotating machinery prognostics: State of the art, challenges and opportunities, Mechanical Systems and Signal Processing, 23, 2009, pp. 724-739, A, Messaoud, C. Weihs, and F. Hering, Detection of chatter vibration in a drilling process using multivariate control charts, Computational Statistics & Data Analysis, Vol. 52, 2008, 3208-3219, A. A., Junior, F. C. Lobato de Almeida, Automatic faults diagnosis by application of neural network system and condition-based monitoring using vibration signals, Proceedings of the 2008 IAJC-IJME International Conference, ISBM 978-1-60643-379-9, and A. G. Rehorn, J. Jiang, P. Orban, State-of-the-art methods and results in tool condition monitoring: review, Int J. Adv. Manuf Technol, 26, 2005, pp. 693-710. Unfortunately to date none of these attempts satisfactorily address the problems caused by chatter in doctor blades.
Thus there is clear need and utility for methods, compositions, and apparatuses for the detection and prevention of chatter in doctor blades. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR .sctn.1.56(a) exists.